Abstract
PIN-FORMED1 (PIN1)-dependent polar auxin transport is essential to organ formation. Previous studies on PIN1 localization uncovered a two-step process in organ formation. (1st) PIN1 polarity converges toward organ initiation sites and auxin accumulates there. (2nd) PIN1 polarizes basally in the center and auxin flow sinks. We have identified a NPH3-like protein MAB4 and its homologs MEL1/MEL2, involved in polar auxin transport in organogenesis. To elucidate the function of MAB4 family genes in PIN1 polarization, we analyzed PIN1 localization in pin-like inflorescences of mab4 mel1 mel2. In the mutant, PIN1 polarity converged normally, but followed basal shift of PIN1 polarity did not occurred, suggesting that MAB4 family genes establish an auxin sink through the control of PIN1 polarity in the meristem. In addition, it is possible that auxin accumulation and its signaling induce the MAB4-dependent basal shit of PIN1 polarity, which is the transition from the 1st step to the 2nd step. To confirm the possibility, we examined the relationship between auxin signaling and MAB4 family genes. Our results indicate that MAB4 family genes function downstream of auxin signaling.
